1. Field of the Invention
This invention relates to methods and apparatus used for the decontamination of enclosed spaces such as pharmaceutical clean rooms, isolators and hospital wards.
2. Present State of the Art
WO-A-2006/031957 discloses a flash vaporizer which provides a constant flow of vaporized hydrogen peroxide or other antimicrobial compounds for rapidly sterilizing large enclosures such as rooms or buildings. The vaporizer includes a heated block which defines an interior bore or bores. The flowpath created by the bore or bores increases in cross sectional area as the hydrogen peroxide passes through the block to accommodate the increase in volume during the conversion from liquid to gas. The vapor is injected into dry air in a duct that circulates it to the large enclosure.
US-A-2006/0008379 discloses a system for microbially and/or chemically decontaminating a room such as hotel room and includes a vapor generator which supplies a decontaminant vapor, such as hydrogen peroxide vapor to the room. The room is then aerated to a level at which it is safe for normal occupants to enter. By using a two step aeration, with a second step at lower humidity than the first, the concentration of residual hydrogen peroxide is reduced rapidly to safe levels of 1 ppm or less, typically about 0.5 ppm, in under four hours. The room is rendered substantially free of contaminants, such as those responsible for Severe Acute Respiratory Syndrome (SARS), Norwalk virus, and unpleasant odors.
EP-A-1655041 discloses a mobile decontamination vehicle comprising a vessel containing a supply of hydrogen peroxide; a catalytic reactor coupled to the vessel to receive a first flow of hydrogen peroxide and to at least partially decompose the hydrogen peroxide into decomposition products. A jet of said decomposition products is directed in the opposite direction to the required direction of travel of the vehicle to provide a thrust to propel the vehicle. A supply of undecomposed hydrogen peroxide is carried in the apparatus for'use at a location to be decontaminated.
Vapour phase bio-decontamination is generally a four phase process. During the first phase the relative humidity inside the chamber is brought to a pre-set value. This is followed by the second phase during which the active vapour concentration inside the chamber is raised to the required value. The next phase is to maintain the active vapour concentration inside the chamber for a sufficient period of time to ensure that bio-decontamination is achieved. The fourth and final phase is to remove the active vapour from the chamber generally by dilution with clean air.
The most commonly used vapour for bio-decontamination is hydrogen peroxide which is generated by “flash” evaporating an aqueous solution of about 30 to 35% w/v. The usual technique for producing the “flash” evaporated vapour is to drop the aqueous solution onto a heated plate held at a temperature above the boiling point of the liquid thus generating a vapour with the same weight ratio as the source liquid. There are two theories as to the action of the hydrogen peroxide; the earlier thinking was that the vapour should be maintained at a concentration below the dew point thus avoiding condensation, the other theory suggests that condensation is necessary to give a rapid bio-decontamination.
There are numerous patents covering the use of gaseous and vapour phase decontamination of enclosed spaces the most important of which are U.S. Pat. No. 5,173,258 and U.S. Pat. No. 7,014,813 B1.
U.S. Pat. No. 5,173,258 describes a single loop closed system in which the carrier gas is circulated from the vapour generator to the chamber to be bio-decontaminated and then back to the vapour generator. On returning to the vapour generator the carrier gas and vapours pass through a device to remove the active vapour and the water vapour thus allowing more hydrogen peroxide to be evaporated into the circulating carrier gas.
U.S. Pat. No. 7,014,813 describes a similar process but has a bypass loop inside the vapour generator. Thus the vapours are not removed from the circulating carrier gas on returning to the vapour generator during the second and third phases of the cycle. This allows a more rapid build up of vapour concentrations and is normally used in cycles when condensation is required.
In both types of bio-decontamination cycles (in which condensation is to be avoided or encouraged respectively) it is essential that the active vapours are distributed evenly throughout the chamber. In some systems the vapours are delivered from rotating nozzles at a velocity up to 20 m/sec and in others external fans are used to move the vapour mixture around the chamber. EP-A-1487503 discloses a portable apparatus for decontaminating an enclosed room or other space comprising a passageway having an air inlet at one end, an outlet at the other end and a pump to cause flow of air through the passageway from the inlet to the outlet. A heater heats the air flowing through the passageway to a predetermined temperature and a flash evaporator is in communication with the passageway. Liquid decontaminant is pumped from a supply of decontaminant to the evaporator to be evaporated and for the evaporant to be delivered to the air flow in the passage to flow in the air flow from the outlet to the rooms to be decontaminated. A universally rotating nozzle is provided at the outlet to distribute the decontaminant containing air throughout the enclosure.
The difficulty in achieving good vapour distribution arises because the air flow through the “flash” evaporator is limited firstly by the size of the evaporator and also because the carrier gas must be heated in order to avoid condensation in the delivery system. The use of rotating nozzles that produce high velocity streams of vapours is of limited value because the mass flow is small and hence the velocity of the plume will rapidly fall as it moves from the source. The addition of fans at different positions in the room is not ideal as their position may be critical to the success of the process and they add complexity to the set-up and management of the equipment.
The final stage of the bio-decontamination cycle is aeration in which the hydrogen peroxide is reduced to a safe level; two techniques in general use for the removal of the hydrogen peroxide from the chamber. The most common is to circulate the carrier gas containing the hydrogen peroxide through a catalyst bed reducing the hydrogen peroxide to water vapour and oxygen, two harmless chemicals. The carrier gas emerging from the catalyst is also commonly dried to remove the water vapour. This purified carrier gas is then returned to the chamber where the hydrogen peroxide concentration is reduced by dilution. The flow rate through the catalyst will determine the time necessary to reduce the hydrogen peroxide to an acceptable value. The vapour generator's catalyst and dryer will have a limited flow for the reasons given above and hence the time required to remove the hydrogen peroxide is likely to be long this is sometimes overcome by having a separate unit to remove the hydrogen peroxide and water vapour. In some rooms fitted with air conditioning systems it is possible to use the air supply and extract to supply fresh air to the room and hence speed up the aeration process.